Stereotactic surgery is that branch of neurosurgery that uses a special device to direct a surgical instrument such as a cannula, electrode or other type of probe or device, with great accuracy, to a target within the central nervous system, particularly the brain or spinal cord, of a patient. The target is located and identified by one of a number of techniques. Sometimes the target can be visualized on computed tomography (CT) or magnetic resonance imaging (MRI). Other times, the position of the target must be determined by its relationship to an anatomic structure that can be seen on scanning using conventional radiographs or by ventriculography. Today, three-dimensional imaging techniques such as CT and MRI are the most frequently used techniques to locate and identify targets within the central nervous system.
Stereotactic surgery can only be carried out using a special apparatus which enables the surgeon to guide surgical instruments to identified targets within the brain, spinal cord, or other part of the central nervous system. There are several such devices in use today ranging from custom designed devices to commercially available devices. Most stereotactic devices have several things in common. The devices are attached securely to the patient, generally the head, the exact three-dimensional spatial relationship between the target and the device is determined after visualization of the target by an x-ray or imaging technique, and the device has a probe or instrument holder that can be adjusted to advance an instrument from various directions to the target with great accuracy. A particularly accurate instrument holder is known as a microdrive which is used by a surgeon to accurately advance a surgical instrument to a predetermined target in the brain.
The commonly used and/or known stereotactic apparati are the Leksell apparatus, the Riechert-Mundinger Apparatus, the Todd-Wells apparatus and the Brown-Roberts-Wells apparatus. The Leksell apparatus is a target-centered device utilizing a 19 cm radius arc and traveler arrangement to guide a surgical instrument to the target. The traveler arrangement may be a microdrive. The entire apparatus is generally attached to the patient's head using a metal frame.
The Ricchert-Mundinger apparatus is a polar coordinate device with two movable arcs allowing motion of the probe holder such as a microdrive in three dimensions. Once aligned with target point-entry point vector, the probe length need only be determined for the proper distance to reach the target point.
Like the Leksell apparatus, the Todd-Wells apparatus is a target-centered device in which the target point is placed at the center of an adjustable radial coordinate system. The Todd-Wells apparatus differs from the Leksell apparatus in that it is the head which is moved to align the target point, not the frame.
The Brown-Robert-Wells apparatus was the first device created for use with CT. The apparatus established the concept of frame-centered sterotoxy, operating in such a manner so that the target need not be moved to the center of some arc. This was possible by making use of the three-dimensional information in the CT images to establish a three-dimensional vector of entry point to target point. The frame is then adjusted by moving four rotational settings so that the probe holder aligns with the entry-target vector.
A variety of instruments may be used with any of these or other stereotactic devices including electrodes, cannulas, biopsy instruments, catheters and the like.
A particularly preferred type of probe or instrument holder is a microdrive which attaches to and forms a part of the stereotactic apparatus to provide the capability of accurately guiding and directing a surgical instrument to a target. A microdrive, as opposed to other stereotactic instrument holders is an instrument holder which includes means to accomplish a controlled advancement of a medical instrument.
In use, a 0 point or reference point is determined by an appropriate technique such as CT or MRI and the target point within the brain is calculated in reference to the 0 point. Then, the stereotactic head assembly is adjusted and the instrument holder or microdrive is set with reference to the 0 point. A surgical instrument is then ready to be advanced into the brain.
Many microdrive devices have been developed for use in laboratory animals. For example, Bland et al, "A Direct-Drive, Non-Rotating version of Ranck's Microdrive," Physiology & Behavior, Vol. 24, pp. 395-397 (1990) describes a direct drive, non-rotating microdrive for the implantation of microelectrodes for the measurement of extra cellular unit potentials in freely moving animals. Bland's microdrive utilizes a headed stainless steel screw to move an electrode into a desired location within the animal's brain. This microdrive has only a very limited advancement range and is not adapted for stereotactic frames suitable for human use.
Radionics (Burlington, Mass.) is the only supplier of current commercially available microdrives suitable for use in human subjects. The Radionics.RTM. microdrive utilizes a gear assembly to achieve depth placement. There are inherent drawbacks to such an approach. First, actuating the gear mechanism causes undesirable vibration along the instrument, which could result in vibration of the implement being lowered into the burr hole; e.g., a cannula. This can result in undesirable tissue damage. In addition, the mechanical engagement of the gear teeth may cause some debris to flake off the instrument, potentially contaminating the operation site. Further, some surgeons have found that the instrument tends to "free fall," which could cause sudden, uncontrolled movement of the cannula into the brain of the patient.
U.S. Pat. Nos. 5,004,457 and 5,006,122 disclose tissue implantation systems which utilize a conventional instrument holder available from David Kopf Instruments (Tujanga, Calif.).
Accordingly, there is a need for a microdrive which will provide easy setting of zero points with reference to a patient's scalp, skull, or dura, as the case may be. In addition, there is a need for a microdrive which can also capture a pusher which is often used to hold an implant in position while a cannula or other instrument is being removed from a target.
It is an object of the present invention to develop a microdrive that overcomes the disadvantages of the prior art microdrives and which is useful with a variety of stereotactic devices used for human surgery.